Indicate fast train (7)
I believe the answer is:
express
I believe this clue is a double definition.
'indicate' is the first definition.
(thesaurus)
'fast train' is the second definition.
(an express train)
(Other definitions for express that I've seen before include "Show" , "for a specific purpose?" , "Train; utter" , "Of train, travelling rapidly with few or no stops" , "Swift" .)